A hard disk drive was originally developed as a storage device of a computer. In recent years, however, the hard disk drive is extensively employed not only in the computer but also in home electric appliances such as a video recorder, a portable music player and a camcorder.
Expansion of use of the hard disk drive is sustained by the increased storage capacity and the reduced price. The storage capacity of a hard disk drive has been increased one hundred times for the last decade, whereas the price thereof has been decreased steadily. There is a strong demand for a device capable of storing enormous information in a cost-effective manner, which tendency is expected to last in the future. Under the circumstances, high accuracy and low price are all required in each part of the hard disk drive. A rotor hub, which is a subject matter of the present invention, does not constitute an exception to this tendency.
A dedicated special motor called a spindle motor is used in a hard disk drive 10 shown in FIG. 11. The spindle motor 11 plays a role in rotating hard disks 14 in the hard disk drive 10. Seeing that information is recorded on the hard disks 14 in an extremely high density, i.e., in a quite narrow interval, it is necessary for the spindle motor 11 to rotate the hard disks 14 with no vibration as far as possible. For that reason, the spindle motor 11 has a unique construction. For example, a high precise bearing called a fluid dynamic bearing is employed as a bearing for supporting a rotating shaft. No matter how accurate the bearing is, that alone is not sufficient to stably support the hard disks 14.
A rotor hub 21 plays an important role in stably supporting the hard disks 14. The spindle motor 11 and its neighboring portions shown in FIG. 11 are enlargedly illustrated in FIG. 12. The rotor hub 21 is supported by a bearing mechanism having a shaft 20 and a sleeve 22, thereby forming a rotating part. The hard disks 14 can be mounted on the rotor hub 21. The hard disks 14 are placed on a disk mounting surface 25 of the rotor hub 21 and are fixed to the rotor hub 21 by means of a clamp 28, with a spacer 27 interposed therebetween. If there is any eccentricity or strain in the rotor hub 21, it may be a direct cause of vibration of the hard disks 14. In this regard, the working accuracy of the disk mounting surface 25 is of particular importance. Accordingly, a highest degree of working accuracy is required in the rotor hub for use in the spindle motor. Taking this into account, the rotor hub has heretofore been manufactured through machining, e.g., cutting, which is known to provide a high degree of working accuracy.
However, cutting is costly to perform. In order for the hard disk drive to become cheaper and to be used more widely in the world, it is essential to reduce the manufacturing costs of the rotor hub. Examples of a cost reduction method include the methods disclosed in Japanese Patent Laid-open Publication Nos. 2003-285228 and 2005-205499. In these methods, a rotor hub of rough shape is formed by plastic working such as cold forging or the like and then finished by cutting or grinding. Use of the plastic working in combination makes it possible to reduce the cutting quantity of a material and to enjoy increased cutting accuracy.
With the two conventional methods mentioned above, it is possible to reduce the cutting quantity of a material but it is not easy to perform the cutting operation after the forging work. In particular, it is difficult to secure the accuracy of a disk mounting surface.
The present inventors made an attempt to use free-cutting stainless steel as a raw material of a rotor hub in an effort to assure easier cutting. If the free-cutting stainless steel is merely used in the two conventional methods, it is often the case that the stainless steel is cracked during the forging process. Furthermore, since the rotor hub is manufactured by plastic working, there is a fear that inclusions contained in the free-cutting stainless steel become easily separable. When separated, the inclusions adhere to hard disks. Such adherence of the inclusions is not permitted because the surfaces of the hard disks need to be kept clean.